The present invention is generally related to analog circuits, and more particularly to a circuit and method for setting the transconductance of a transconductor.
MOS (metal oxide semiconductor) is a widely used semiconductor technology having relatively low power requirements, making it particularly attractive for use in battery-powered devices, such as cellular phones and portable computers, for example. In MOS technology, the speed of a transistor is related to the mobility of the electrons in the channel of the transistor. Mobility is a physical constant that depends on the temperature. For a 100 degree C degree variation in temperature, mobility can decrease by as much as a factor of 2, for example. This places a large constraint on circuit designers, who must design circuits that operate over a wide range of temperatures.
As an example, filters are frequency-selective circuits capable of passing to the output only those input signals that reside in a desired range of frequencies. Filters are particularly sensitive to temperature changes because they are tuned to operate over a certain range of frequencies.
What is needed in the art is a circuit and method for solving the prior art problem of mobility variations in filter circuits resulting from changes in temperature.
The present invention achieves technical advantages as a circuit and method for setting the transconductance of a differential transistor pair of a transconductor to a specific value, regardless of temperature, while maintaining the gate overdrive of the differential transistor pair within a specific range to optimize performance.
In one embodiment, disclosed is a transconductance-setting circuit, including a first transconductor coupled to a reference voltage and adapted to produce a current output. A reference current source is coupled to the first transconductor, and a feedback loop is coupled to the first transconductor and the reference current source. The feedback loop is adapted to reduce error in the current output and set the transconductance of the first transconductor to a value proportional to the ratio of the reference current and the reference voltage.
Also disclosed is an input filter stage, including a first transconductor coupled to a reference voltage and adapted to produce a current output. A reference current source is coupled to the first transconductor, and a feedback loop is coupled to the first transconductor and the reference current source, where the feedback loop is adapted to reduce error in the current output and set the transconductance of the first transconductor to a value proportional to the ratio of the reference current and the reference voltage.
Further disclosed is a method of setting the transconductance of a first transconductor. The first transconductor is driven by a reference voltage, and is coupled to a current reference and a feedback loop. The method includes the steps of applying the reference voltage to the first transconductor, comparing the current output from the first transconductor to the reference current, and setting the difference between the first transconductor current equal to the reference current with the feedback loop.
The present invention provides a temperature-independent constant transconductance with tight tolerancing of the gate overdrive voltage range. The tuning scheme of the present invention is robust and very accurate, with transconductance errors of less than +/xe2x88x920.2% being achieved. The transconductance of a differential transistor pair remains constant regardless of process variations and is also useful in the design of constant gain-bandwidth-product op-amps. The invention works well in any technology, and is particularly useful for mixed signal or analog circuits.